Many natural and synthetic compositions may benefit from additives that modify rheology. For example, lubricant oil formulations generally contain viscosity index (VI) improvers derived from polyolefins that modify rheological behavior.
There have been many attempts to develop polyolefin additives that have a high thickening efficiency without raising the average ethylene content or the propensity to chain scission under shear.
However, conventional polyolefin additives suffer from unfavorable characteristics such as: (a) a high molecular weight fraction such that they are more affected by shear induced degradation of the molecular weight—such compositions have an unfavorable Thickening Efficiency (TE)/Shear Stability Index ratio (SSI) ratio in that they have a lower TE for a given SSI; (b) preparation with metallocene catalysts in bulk polymerization process, which provides process reactor heterogeneity that leads to significant intermolecular composition and broadening of polydispersity index; (c) a blend of amorphous and semi crystalline polyolefins that have a significant and predetermined intermolecular compositional heterogeneity; and (d) polymerization conditions providing polymers having significant long chain branching, which provides a diminished TE because they are topologically constrained from being dispersed uniformly, at a molecular level, in solution.
Conventional VI improvers are taught in U.S. Pat. Nos. 4,540,753; 4,804,794; 4,871,705; 5,151,204; 5,391,617; 5,446,221; 5,665,800; 6,525,007; 6,589,920; and 7,053,153, which are each incorporated herein by reference in their entirety.
Some conventional VI improvers, such as those described in U.S. Pat. Nos. 4,540,753 and 4,804,794, use an ultra narrow Polydipersity Index (PDI) composition. It is anticipated that these ultra narrow PDI polymers lack a high molecular weight fraction so that they would be less affected by the shear induced degradation of the molecular weight. Such compositions are expected to have low SSI or a correspondingly high TE/SSI ratio.
Other conventional VI improvers, such as those described in U.S. Pat. Nos. 4,871,705 and 5,151,204, attempt to overcome structural limitations by using a metallocene catalyst which provides a polyolefin having a distribution of molecular weights. However the use of the metallocene catalysts in bulk polymerization process as described in the applications indicates that the process reactor heterogeneity would lead to significant intermolecular composition and broadening of the polydispersity index in the copolymer. Without being limited by theory, it is believed that the broader polydispersity index is due to differences in the mixing and transport and equilibration of the constituent monomers as well as differences in the temperature of the different positions inside the polymerization reactor.
Another conventional VI improver includes a blend of amorphous and semi crystalline polyolefins as described in U.S. Pat. Nos. 5,391,617 and 7,053,153. The combination of two such polymers attempts to provide increased TE, shear stability, low temperature viscosity performance, and pour point. However, the design of the molecules have a significant and predetermined intermolecular compositional heterogeneity.
Still another conventional VI improver is described in U.S. Pat. Nos. 6,525,007, 6,589,920, and 5,446,221. Such compositions are prepared with a single site metallocene catalyst in a solution polymerization. However the choice of the metallocene catalysts as well as the polymerization conditions indicate that these polymers should have significant long chain branching as shown in U.S. Pat. No. 5,665,800. Such long chain branched polymers have a diminished TE compared to their linear analogues since they are topologically constrained from being dispersed uniformly, at a molecular level, in solution.
There remains a need for VI improving compositions that promote the following in lubricant oils, while maintaining a low ethylene content: (a) a more constant viscosity over a broad range of temperatures; (b) improved TE; and (c) improved ratio of the TE to the SSI.